This SLiMM proved to be more efficient than a comparable palladium membrane. Source: WPI/Curtis Sayers

Cars powered by hydrogen fuel cells have a longer range than electric vehicles, a lower overall environmental impact and can be refueled in minutes. The motors they run on would produce only water as a waste product. So why aren’t we driving them?

One reason is that producing, distributing and storing the pure hydrogen needed to power these vehicles is costly and complex. Despite the fact that hydrogen is the most abundant element in the universe, it is almost always chemically bound to other elements — so pure hydrogen is more difficult to come by than, say, water (where hydrogen is too busy being tied up with oxygen to be available for the filling station).

Nearly all of the hydrogen produced in the U.S. is obtained from hydrocarbon fuels such as natural gas, through a multi-step process called steam reforming. In the presence of a catalyst, hydrocarbons react with high-temperature steam to produce carbon monoxide, carbon dioxide and molecular hydrogen. A membrane, typically made with the precious metal palladium, can then be used to isolate the hydrogen. Palladium has a high hydrogen solubility and permeance—but is also expensive and fragile.

Chemical engineers have long searched for alternatives to palladium, without success. But, a new study led by Ravindra Datta, professor of chemical engineering at Worcester Polytechnic Institute (WPI), has identified liquid metals as a potential substitute.

As published in the Journal of the American Institute of Chemical Engineers, the WPI study shows that, compared to palladium, membranes made from liquid metal appear to be significantly more effective at separating pure hydrogen from other gases. They are also not prone to defects and cracks, and would potentially be far less expensive to manufacture.

Datta’s exploration of the liquid metal alternative began several years ago, picking up where retired WPI chemical engineering professor Yi Hua "Ed" Ma left off. Ma had developed a process for conserving the amount of palladium needed to make a membrane — but its fragility remained a challenge. After reviewing the literature and finding no previous research, Datta successfully applied for a $1 million award from the U.S. Department of Energy to study the feasibility of using liquid metals for hydrogen separation.

Gallium, a nontoxic metal that is liquid at room temperature, showed promise for Datta’s team — but also presented the challenge of reactivity with the ceramic materials commonly used as palladium membrane supports. Modelling and experimentation led to alternative materials, as well as a new support structure — a SLiMM, or sandwiched liquid-metal membrane. A membrane consisting of a thin layer of liquid gallium between a layer of silicon carbide and a layer of graphite proved to be up to 35 times more permeable to hydrogen.

"These tests confirmed our hypotheses that liquid metals may be suitable candidates for hydrogen separation membranes," Datta said. “There are a host of questions that still need to be answered, including whether the small membranes we constructed in the laboratory can be scaled up, and whether the membranes will be resistant to substances present in reformed gases (including carbon monoxide and sulfur) that are known to poison palladium membranes.”

Besides gallium, according to Datta, there are many other metals and alloys worth exploring. “We have opened the door to a highly promising new area of hydrogen energy research,” he said. “It is a vast open field.”